1. Field
The present disclosure relates to poly(benzimidazole-co-benzoxazole) and a method for preparing the same. More particularly, the present disclosure relates to poly(benzimidazole-co-benzoxazole) (PBI-co-PBO), a polymer electrolyte including the same and a method for preparing the same, a polymer electrolyte membrane including the polymer electrolyte and a method for producing the same, and a membrane electrode assembly and a fuel cell using the polymer electrolyte membrane.
2. Description of the Related Art
Polymer electrolyte membrane fuel cells (PEMFC) capable of being operated at high temperature enhance catalytic activity while reducing the poisoning of a catalyst caused by carbon monoxide, and thus are suitable as fuel cells (Non-patent Documents 1-3).
Perfluorosulfonate polymers have been used widely as polymer electrolyte materials for polymer electrolyte membrane fuel cells. Although such perfluorosulfonate polymers have high chemical, mechanical and thermal stability and high proton conductivity, they show high proton conductivity only when they are humidified with water. Since the proton conductivity significantly depends on water content as mentioned above, the proton conductivity may be decreased, for example, when water evaporates at 100° C. or higher, resulting in degradation of the quality of a cell (Non-patent Documents 4 and 5).
Thus, many studies have been conducted to develop materials having high ion conductivity even under high temperature low-humidity or anhydrous conditions. Recently, intensive studies have been conducted about some electrolytes capable of conducting protons even under dry conditions.
For example, it has been reported that CsHSO4 and CsH2PO4 show high conductivity approximately at 110° C. (Non-patent Document 6). Heteropolyacids have also been developed as fuel cell electrolytes. It has been found that heteropolyacids show high proton conductivity even under low relative humidity at room temperature (Non-patent Document 7). In addition, it has been reported that combinations of ammonium difluoride with different ammonium salts may be used as protic electrolytes in various temperature ranges (Non-patent Document 8). Further, acid-base composite protic ionic solutions containing organic amines or strong acids in ionic solution show high proton conductivity without humidification (Non-patent Documents 9 and 10).
Meanwhile, phosphoric acid having unique properties of thermal stability and high proton conductivity is applicable to fuel cells. Phosphoric acid has a proton conduction path similar to the Grottuss mechanism, and thus may provide high proton conductivity in the absence of water even at a high temperature more than 100° C. (Non-patent Document 11).
Therefore, many studies have been conducted to develop a phosphoric acid-doped poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI) membrane as an electrolyte material for a high-temperature polymer electrolyte membrane fuel cell (Non-patent Documents 12-19). Such phosphoric acid-doped polybenzimidazole requires no water to provide proton conductivity. Particularly, an acid-base composite having a strong acid, such as phosphoric acid, and PBI shows high proton conductivity. It is thought that this results from the hop and turn mechanism of protons and phosphoric acid molecules (Non-patent Documents 12 and 16).
A gel-like phosphoric acid-doped polybenzimidazole membrane has been produced by a polyphosphoric acid process (Non-patent Documents 19 and 20). In the corresponding process, a polybenzimidazole membrane is obtained in-situ from a polymerizable solution mixture. The membrane has high proton conductivity due to a high acid doping level, but is problematic in that it shows low mechanical strength. In addition, when the membrane is obtained in the form of a gel-like polymer matrix, it causes a significant drop in mechanical strength due to degradation of tensile strength (Non-patent Documents 18, 21 and 22). In general, the in-situ polybenzimidazole membrane has higher proton conductivity but lower mechanical strength as compared to the membrane not obtained by the in-situ process due to a high phosphoric acid doping level (Non-patent Document 21).